1. Field of the Invention
The present invention relates to a method of manufacturing an embodiment of a measure. The embodiment of a measure includes a bending resistant graduation carrier member and a base member and fastening elements for fastening the graduation carrier member on the base member. The graduation carrier member and the base member have different temperature expansion coefficients.
1. Description of the Prior Art
Embodiments of measure of this type are used, for example, in longitudinal measuring devices in machine tools. Measurement operations cause problems in machine tools because machine, longitudinal measuring device and tool are of different materials and, therefore, rising temperature during the operation of the machine tool leads to different dimensional changes of the various components.
Since, for various reasons, the machine component and the scale and the housing for the scale will rarely be of materials having the same thermal expansion coefficient, the unavoidable temperature changes will cause different length changes, particularly of the machine component and the scale. Thus, inaccuracies of the measurements will occur which are unacceptable in view of the measurement accuracies demanded today. Most frequently combined are machine components of gray cast iron, a housing of aluminum and a scale of glass or steel.
The above-described problems have been known for a long time and many solutions have been suggested for reducing by means of so-called temperature compensation the errors which are due to the temperature-related dimensional changes. In the following, those suggestions are discussed in which the compensation is carried out by means of an embodiment of a measure.
German Utility Model No. 7,513,496 describes a compensating device in which, in addition to the temperature-related error, inaccuracies of the machine are compensated. A glass scale is cemented into a hollow aluminum section with a highly elastic layer being placed between the scale and the hollow section. The hollow section is fastened to the machine by means of special end pieces. The highly elastic layer serves to mechanically uncouple the glass scale from the hollow aluminum section. Thus, constraining forces cannot be transmitted from the machine to the glass scale. The compensating device acts on the end faces of the glass scale and can compress or stretch the glass scale in accordance with the requirements of the error pattern. Accordingly, the compensating device adjusts the effective length of the graduation.
The error compensation of the type described above has been found useful. In addition, the fastening of a scale in a measuring device in a floating manner has been known at least since German Pat. No. 1,176,382. The fastening in a floating manner is now considered necessary in order to fasten the highly accurate graduation free of constraining forces. A number of other publications show that it has been considered necessary in the art to uncouple the graduation from the machine, so that no thermally related constraining forces influence the graduation. This view is expressed, for example, in German Offenlegungsschrift No. 2,016,253 and U.S. Pat. Nos. 3,816,002 and 3,629,945.
More recently, those skilled in the art have gradually changed this view and have rigidly connected embodiments of measure to machines. This more recent view is expressed, for example, in German Offenlegungsschrift No. 3,419,527. In this reference, an embodiment of measure is calibrated by means of calibrating screws at a normal temperature of, for example, 20.degree. C. When the measuring device is mounted on the machine, initially the temperature at the mounting surface is measured. The embodiment of measure is then adjusted in accordance with the temperature of the mounting surface by means of the calibrating screws and, thus, is calibrated with respect to the machine. Subsequently, the position measuring device is rigidly mounted on the machine.
Due to the rigid fastening of the two ends of a scale on the machine, the machine as well as the scale are subjected to the same longitudinal deformation when temperature changes occur, so that temperature-related inaccuracies of measurement can no longer happen in working a workpiece. However, in the case of bending resistant scales of glass, this proposal can be used only to a relatively limited extent because, when the temperature rises significantly above the normal temperature, the base member on which the graduation carrier member is mounted expands to a greater extent than the glass scale, so that the glued connections at the end faces tear.
It is, therefore, the primary object of the present invention to provide an embodiment of a measure which carries out the temperature-related longitudinal deformations of the machine even when heated substantially above normal temperature.